A plurality of adjacent power stages can be set in heaters currently used for interior spaces of vehicles, especially the driver's cabs of trucks. In general, a thermostatic monitoring of the room temperature as well as automatic regulation of the heat output are provided, and, depending on the variance of the room temperature, different power stages of the heater are switched on by an automatic unit, or the latter unit performs a changeover between the power stages in order to bring the room temperature to the set value or to maintain it at the set value. If the doors or windows of the vehicle space remain closed for a long time or remain open without change (and not excessively), a final stage, in which a high-low control takes place with more or less rapid change between adjacent power stages and the room temperature remains constant within relatively narrow limits, will become established after a starting phase of variable duration.
At all power stages, the air and fuel throughputs are essentially such that a relative optimum of consumption is obtained with respect to the fuel needed. Different power stages therefore differ regularly in terms of both the fuel throughput and the air throughput and consequently also in terms of the output of the burner blower, which is primarily responsible for the air throughput. For reasons of comfort, the air throughput and the fuel throughput are not changed, in general, abruptly at the time of a changeover between power stages, but a phase of transition of varying duration, during which the values of the air and fuel throughputs are adjusted to the standard values of the power stage currently being set more or less continuously and with a time delay of variable duration, is passed through after a changeover between the power stages. Concerning the output of the blower of the burner, which maintains the air throughput, this means that the blower output is raised or lowered "gradually" during the said phase of transition at the time of a changeover between heating output stages.
The noises of the blower may also be felt to be a nuisance, especially when the driver spends a rather long time in the parked vehicle with the engine not running during the nighttime hours or during prolonged breaks.